The present invention relates to a method and arrangement for starting a slip-ring machine, the arrangement comprising a frequency converter arranged to be connected between a supply network and a rotor of the slip-ring machine for controlling the slip-ring machine, and starting means for starting the slip-ring machine.
The use of slip-ring machines has recently quickly become increasingly common in connection with high-powered drives in particular. Examples of such drives include wind generator drives and motor drives requiring a high torque. In a variable frequency use, slip-ring machines are typically controlled employing a “doubly-fed” principle. In such a manner of control, a stator of the machine is connected to a three-phase network whose frequency is constant. The network connected to the stator operates as a supply network when the slip-ring machine serves as a motor and, correspondingly, as a network to be supplied when the slip-ring machine serves as a generator.
In a doubly-fed use, the rotor of a machine is connected through slip rings to a frequency converter which is further connected to a supply network or to a network to be supplied. The frequency converter enables the rotor to be magnetized almost arbitrarily. A rotating magnetization to be provided for the rotor enables the operation of a doubly-fed machine to be controlled and its power and power factor to be set both in generator as well as in motor mode. A method of controlling a drive is disclosed in U.S. Pat. No. 6,448,735.
A frequency converter of a rotor circuit is dimensioned to operate typically within a speed range of approximately ±30% in the environment of a synchronous speed determined by the frequency of a network and the number of pole pairs of a machine. In the doubly-fed use, the greatest advantage is achieved in the inexpensiveness of the frequency converter equipment since its capacity is to be only about one fourth of the shaft power of the machine if the range of the controlled speed lies in the environment of the synchronous speed. When a controlled speed range lies ±30% from the synchronous speed, the frequency converter is only to supply approximately 100%×30%/(100%+30%)=23% of the shaft power of the machine at a maximum speed if the losses of the machine are ignored.
In connection with most applications, such as pumps and blowers, restricting the speed control is not a problem. A low rotation speed is used only during starting and stopping.
The fact that such doubly-fed slip-ring machines are difficult to start has prevented them from being largely used also in drives other than generator drives. When a machine is not running, the voltage induced into a rotor winding is about three times with respect to the rated voltage of a frequency converter to be connected to a rotor circuit, so the frequency converter cannot be connected directly to the rotor circuit. Previously this problem has been solved by using a starting resistor in the rotor circuit. The winding of a rotor is then provided with a resistive circuit through slip-rings. When the machine has accelerated close enough to the synchronous speed, the frequency converter may be connected to the rotor circuit and the resistors may be removed. FIG. 1 shows a prior art implementation of a starting arrangement for a slip-ring machine.
A problem with the resistor starting is a decrease in the torque of a motor as the motor accelerates. Therefore, the resistance of a rotor resistor is decreased usually by short-circuiting the starting resistor one part at a time. However, such a solution is complex, requiring numerous contactors which, in more high-power devices, are large and expensive. In addition, each short circuit causes a transient in the torque of the motor, which strains the rotating equipment.